Superadsorbent material system for improved filtration applications

ABSTRACT

The present invention involves the use of nanoporous carbons derived from partially or fully demetalized metal carbides in personal protection equipment for the irreversible absorption/adsorption of both broad and specific targeted gases. These materials have been specifically processed to provide enhanced effective loadings against specific harmful volatile organic compounds.

RELATED APPLICATIONS

This application claims the benefits of U.S. Provisional Application No.61/681,272, filed Aug. 9, 2012 which is herein incorporated by referencein its entirety for all purposes.

FIELD OF INVENTION

The present invention relates to providing personal and publicprotection through enhanced filtration, and more particularly to amedium developed for the irreversible absorption or desorption oftargeted gases. The medium is processed to yield a form factor (e.g.,particle type, particle size, surface area, pore size, pore sizedistribution, total pore volume, functionality, etc. . . . ) specific tothe capture of a given gas (e.g., through use in chemical respirators).

BACKGROUND OF INVENTION

Current materials do not provide adequate ventilation protection againstselect chemical agents, toxic industrial compounds, toxic industrialmaterials, and other harmful volatile organic compounds. These chemicalshave very low physical exposure limits yet are difficult to capture andretain in personal or collective filtration devices. This lack ofperformance leads to large, heavy respirator mask canisters that impedewar fighter and emergency responder performance and field of view.Furthermore, the large mass and volume of current material requiredleads to a large pressure drop across the bed resulting in laboredbreathing in order to pull sufficient air through the mask forrespiration.

A need therefore exists for an improved adsorbent material in personalprotection equipment to irreversibly adsorb targeted gases from theenvironment.

SUMMARY OF INVENTION

The present invention involves the use of nanoporous carbons derivedfrom partially or fully demetalized metal carbides in personalprotection equipment for the irreversible absorption and adsorption ofboth broad and specific targeted gases. These materials have beenspecifically processed to provide enhanced effective loadings againstspecific gases. This parameter corresponds to the amount of target gasthe filtering medium is capable of adsorbing: the larger the effectiveloading, the more target gas is “trapped” into the medium. Inparticular, the medium has been modified to present a specific particlesize distribution and to provide a high surface area and a high totalpore volumes. Specifically, this medium was further designed to behighly porous (commonly 2,250 m²/g) with tunable nanoscale pores (1-5nm) of various pore shapes. The functionality that is added for thetargeted adsorption/absorption of gases was also tailored to providehigh effective loading capabilities in dry and wet atmosphericconditions as tested in relative humidity of 0% and 80% respectively.

One general aspect of the present invention is that it is at least seventimes more adsorbent per unit mass than what is currently used. Comparedto the prior art filtration material, the present invention has a muchhigher capacity to filter and retain chemical adsorbents across a broadclass of chemical compounds. The invention also utilizes nanoporouscarbons which are extremely hydrophobic while still able to capturevolatile organic compounds, thus providing practical operation underhigh relative humidity. Furthermore, once adsorbed, the chemicals can bedesorbed to provide analysis of an exposure event.

Those skilled in the art will appreciate that the present invention willsignificantly improve the protection offered to the warfighter throughthe application of such material for personal filtration. For example,the present invention can be implemented as powders and substituted intoexisting gas masks. Alternatively, fibers of the material can be usedand integrated into protective clothing. Finally, the invention can alsobe integrated into building filters for collective protection.

BRIEF DESCRIPTION OF DRAWINGS

The present invention is further described with reference to thefollowing drawings wherein:

FIG. 1 is a graph displaying the gas chromatograph/mass spectrometerdetection of 50 volatile organic compounds simultaneously adsorbed ontoa nanoporous carbon and a standard sorbent.

FIG. 2 is an image displaying the comparative sizes of the standardfiltration mask canister (left) and the nanoporous carbon containingcanister (right).

FIG. 3 is a graph displaying the performance of several differentnanoporous carbons compared to the standard filtration mask.

FIG. 4 is a graphical representation comparing various embodiments ofthe invention using acid, impregnation, and previous embodiments.

FIG. 5 is a graphical representation comparing various embodiments andprior art under alternating drying conditions.

FIG. 6 is a graph comparing breakthrough curves of one embodiment withprior art.

DETAILED DESCRIPTION

Nanoporous carbons outperform other state-of-the-art sorbent materialsin both adsorption rate and capacity for various volatile organiccompounds. The present invention can adsorb a broad range of volatileorganic compounds, including toxic industrial compounds and a limitednumber of chemical warfare agents, even under conditions of 80% relativehumidity.

The present invention is further defined by the following workingembodiments:

Referring to FIG. 1, an experiment was performed to demonstrate thecapability of one embodiment of the present invention to adsorb multiplecompounds simultaneously without significant displacement or poreocclusion. At 7 L/min gas flow over 5 minutes of a 50 component 100 ppbhydrocarbon gas mix, 250 mg of nanoporous carbon outperformed 250 mg ofa standard sorbent, and showed fairly linear adsorption capacity acrossmolecular weights from 44 g/mol to 120 g/mol.

One embodiment of the present invention has established the relevance ofusing nanoporous carbide-derived carbons against difficult to capturetoxic industrial compounds, and thus the capability of reducing the sizeof a filtration mask. In the following working example, baseline fullydemetalized material as well as partially demetalized and surfacefunctionalized materials were compared to the current widely usedpersonal filtration medium for chemical respirators. Activated carbonimpregnated with copper, Silver, Zinc, Molybdenum, andTriEthlyeneEiAmine, (ASZM-TEDA).

Comparison studies between nanoporous carbons and ASZM-TEDA requiredthat a consistent flow rate through a packed bed of material beestablished. Particle size distribution analysis confirmed thatnanoporous carbon particles are about 350 μm in diameter compared to theabout 1.0 to 1.5 mm diameter activated charcoal particles of ASZM-TEDA.As a result of the size difference, packed beds of equivalent massshowed a much lower pressure drop across ASZM-TEDA packed beds ascompared to the nanoporous carbon bed. Testing revealed that equivalentflow rates and pressure drops could be obtained if 200 mg of ASZM-TEDAwas tested versus 30 mg of nanoporous carbon. Therefore, further testingwas conducted using only 30 mg of nanoporous carbon in comparison to 200mg of ASZM-TEDA.

The test conditions for determining the breakthrough point of 40 ppmacetonitrile (ACN) at 5 L/min were devised to yield physiologicallyrelevant data. Given that the average mass Of ASZM-TEDA in a personalfiltration mask is about 120 grams and that the breathing rate undermoderate exercise for an average male is about 30 L/min., our test of200 mg ASZM-TEDA and 5 L/min ACN represents a 1/600th scale model. Inthese conditions, one minute of test gas exposure equates to about 20minutes of field canister use.

The present invention was also tested to show that partialdemetalization and surface functionalization of standard nanoporouscarbon particles will improve the capture of difficult to trapmolecules, such as ammonia.

FIG. 2 shows one embodiment of the present invention a canister 6 ofreduced size. Also portrayed in FIG. 2 is the difference between thisembodiment and the prior art for chemical respirators, the current C2A1canister 4. The difference in canister size is evident and represents anapproximate 85% reduction in material weight and about 80% reduction inoverall volume. The smaller sized C2A1 canister 6 still maintainsequivalent pressure drop across the bed and improves performance by100%. This also allows for easier breathing.

Additionally, because effective desorption when using nanoporous carbonsrequires elevated temperatures, for example, about 325° C., over 5minutes, use of the present invention as a filtration media is highlyattractive since no analyte desorption is expected at standard operatingtemperatures for this embodiment.

Referring to FIG. 3, the data show that for ammonia (NH₃), the ASZM-TEDAmaterial performs slightly better than a standard molybdenum or lowpercent residual titanium nanoporous carbon. However, it is interestingto note that the overall breakthrough profiles for the nanoporouscarbons are much sharper than ASZM-TEDA. This sharper slope indicatesimproved complete protection until the material is defeated. Hydrogentreated nanoporous carbon (MO8HL) with about 5% residual metal performsbetter than AS ZM-TEDA and the surface oxidized sample outperformsASZM-TEDA by almost 300%. This long breakthrough time approaches that ofthe zeolyte ZSM5, which was specifically designed to trap NH₃ and doesnot perform well for other gases of interest.

The data in FIG. 3 was obtained using a 20 mg packed bed of materialwhile flowing 1000 mg/m³ of NH₃ through the bed at a rate of 20 ml/min.These working embodiments were conducted in helium as a heliumionization detector was used to determine the breakthrough curves. Thedisplayed axes were normalized as min/g of material and 100% deliveredgas for comparative purposes and to correct for test to test variabilityin the detector.

Other embodiments focused on improving the functionalization of themedium. Two paths were investigated in parallel: acid treatment andimpregnation by a metal salt. The study on the metal salt impregnationdid not lead to any significant improvement in the effective loadingdespite varying several parameters (salt type, salt concentration,activation of the CDC, etc. . . . ). However, unexpected results wereobtained from the study on acid treatment. During the optimization ofthe acid treatment, it was demonstrated that the performance of afunctionalized Mo₂C is comparable (and in some cases better) to thevalues obtained with the prior art ARC material.

One embodiment of this invention is the process or functionalizationprotocol which utilizes an acid treatment. In one working example ofthis embodiment, 300 mg of Mo₂C was measured and put in a 40 mL vial.Then, 10 mL of nitric acid was added to the vial with Mo₂C. The vial wasequipped with a condenser and placed on a preheated heating block. Afterheating, Mo₂C was filtered through a sintered glass funnel and thenthoroughly rinsed with distilled water until the rinsing water pRbecomes neutral, Mo₂C was then dried under vacuum at room temperature.

FIG. 4 is a graphical representation of the results of a secondembodiment in which the surface of the carbide was activated with theacid treatment developed previously. The results are compared to acidtreatment by itself (0% Fe3Cl sample) and to the impregnation onlyseries (I series). The data indicates that the acid treatment aloneenhances the effective loading at a higher rate than by combining itwith impregnation. In order to investigate potential synergy between theimpregnation and the acid treatment, another series was developedwhereby the acid treatment followed the impregnation (I+A) series. Thedata was compared to the results obtained during the previous experiment(I series). As can be seen, performing the impregnation on activatedcarbide does not improve the effective loading capacity (A+I series).Although performing the acid treatment after the impregnation seems toproduce some effects (especially at a concentration of 20% in metalsalt), it does not surpass the capabilities of samples solely treatedwith acid treatment (0% Fe3Cl sample).

FIG. 5 is a graphical representation of the effects of different dryingconditions on the acidification treatment. In these embodiments, severaldrying conditions were investigated. The first parameter investigatedwas the temperature at which the drying step occurred. Temperaturesvarying from room temperature to 350° C. were tested with a dryingduration of 2 hours. There is a significant improvement in the effectiveloading (30%) when drying the sample at higher temperatures, with asuperior capacity at 240° C. (WE=0.093 g NH₃/g CDC). Higher temperaturesdid not further enhance the effective loading.

FIG. 5 also provides a graphical representation comparing one embodimentof this invention with prior art Ammonia Removal Carbon (ARC). Thiscomparison was performed in order to remove variability due to theexperimental set-up, and therefore establish a baseline number using thecurrent state of the art material in ammonia removal. Upon reception ofthe ARC, it was indicated that the ARC had an effective loading of about0.17 g NH3/g ARC. The data obtained on the set-up for the ARC samplesare reproducible (<1% in standard deviation) and provided and average of0.059 g NH₃/g ARC. Based upon our current testing results, the materialsprovided effective loading ranging from 0.052 to 0.093 g NH₃/g CDC,demonstrating that this embodiment of the invention has, at the leastcomparable, and, at the best, better than capabilities comparable to thestate of the art material.

Although the absolute values of the effective loading are currently indiscussion, the ammonia removal capacity of a material is an intrinsicproperty. Consequently, if two materials are behaving similarly on oneset-up, they should also perform similarly on a second test platform.

FIG. 6 is a graphical representation of a comparison of the breakthroughpoints of one embodiment of this material with the prior art ARC. Theelevated WE for the ARC samples can be attributed to a low slope for thesignal rise while the breakthrough point is relatively early (<1,000min/g of sorbent). Alternatively, the breakthrough point for the presentembodiment is at a greater time (>1,500 min/g of sorbent) andcorresponds to a sharp increase in signal. This difference is criticalin practical terms. As seen in FIG. 6, by the time the presentembodiment materials goes through breakthrough, the ARC material hasalready reached a significant portion of its loading capacity: at least20%-30% when compared to the a first embodiment and up to 80%-90% whencompared to a second embodiment. Consequently, the individuals/soldierswould be breathing an atmosphere free of ammonia for longer period oftimes with gas masks using the second embodiment when compared to gasmasks using the prior art ARC.

In summary, several of the embodiment materials have demonstratedeffective loadings comparable to the effective loading of prior art ARC.One embodiment of the invention has demonstrated an effective loading of0.093 g NH₃/g CDC. Finally, the breakthrough times for the embodimentsare significantly greater than the ones measured for prior art, ARC,demonstrating an enhanced performance over prior art materials.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications andadditions may be made to the described embodiment for performing thesame function of the present invention without deviating therefrom.Therefore, the present invention should not be limited to any singleembodiment, but rather construed in breadth and scope in accordance withthe recitation of the appended claims.

It is claimed:
 1. A method of making a chemical filtration mediacomprising the step of: adding an amount of 5 M nitric acid to an amountof Mo₂C in a container to form a carbide derived carbon; equipping thecontainer with a condenser; heating the container; filtering saidcarbide derived carbon; and drying said carbide derived carbon undervacuum.
 2. The method of making a chemical filtration media of claim 1,wherein the step of heating the container further comprises placing thecontainer on a pre-heated heating block.
 3. The method of making achemical filtration media of claim 1, wherein the step of filtering saidcarbide derived carbon further comprises filtering said carbide derivedcarbon through a sintered glass flannel.
 4. The method of making achemical filtration media of claim 1, further comprising the step of:rinsing with an amount of distilled water until said amount of distilledwater becomes neutral.